Chien’s wife, Nguyen Thi Hien, said some workers called her on April 20 to inform about his death.

“They said the men went to sleep without closing the gas valve, leading to the gas leak that killed all four of them in the room,” she said.

Chien has two young children and Thai has three in Quang Binh. It is unclear about the other two’s marital status.

These workers have worked in Russia for three to four years and often moved to different cities to follow casual jobs.

It has not been confirmed if they were working legally in Russia.

According to latest statistics from the Central Government’s Anti-crime Steering Committee, Russian police bust four cases of forced labor with many Vietnamese workers in 2013. More than 1,000 Vietnamese workers were rescued and repatriated that year.